Add flutter_v2ray_client sample: Flutter VPN & V2Ray Client

[amir-zr]

I would like to contribute a new sample project, flutter_v2ray_client, which demonstrates how to implement a high-performance VPN client using V2Ray/Xray in Flutter.

GitHub Repository: https://github.com/amir-zr/flutter_v2ray_client

Key features of this sample:

• Real-time VPN tunneling in Flutter applications.
• Multi-protocol support (V2Ray/Xray protocols).
• Easy-to-understand, minimal code for educational purposes.
• Includes clear README with setup instructions and examples.

This sample is educational and focused on demonstrating V2Ray integration in Flutter. It does not include the full app functionality, only the core logic needed to understand and implement V2Ray in Flutter projects.

Before submitting the PR, I want to confirm if this type of sample is suitable for the flutter/samples repository.